The present invention relates to a ball worm transmission for transmission of rotational motion between two shafts with nonintersecting axes using rolling elements. It represents a combination of two mechanical principles: the worm transmission and the rolling of spherical balls. The functionality is somewhat similar to a ball-screw mechanism; however it provides a rotational rather than translational output.
The invention resulted from the need for a miniature, kinematically precise, highly efficient rotational transmission with high transmission ratio capable of transmitting relatively high power with no backlash. Such mechanisms are required for actuating revolute joints of precision mechanisms such as small robot manipulators.
Several solutions for creating non-backlash rotational-rotational transmissions (R—R) using gears have been proposed, such as the split gear, the variable pitch (duplex) or conical shaped worm. Each particular approach either presents limited non-backlash torque ranges or is too complex to be miniaturized. Moreover, these solutions present reduced power transmission efficiency (typically on the order of 30%).
For rotational-translational (R-T) transmissions, high kinematic precision and efficiency have been successfully implemented based on the ball-screw mechanism. Ball-screws are readily available on a large variety of sizes and they are widely used for precise mechanisms. The ball-screw mechanism represents a variation of the regular screw mechanism by introducing a number of spherical balls between the screw and the nut and providing a recirculation path for the balls. The screw and the nut are no longer in contact and the motion is transmitted through the balls rolling in between. The key feature that provided the success of this mechanism was the replacement of the sliding friction between the screw and the nut by the rolling friction of spherical balls complemented by a smooth recirculation path implemented into the nut.
The worm transmission was the perfect candidate for implementing the rotational transmission under the required characteristics. It is non-backdrivable and it is relatively simple, thus allowing miniaturization. Its only problem is that it may not be constructed without backlash and it is power inefficient. The classic worm transmission is schematically represented in FIGS. 1A and 1B.
As shown in FIG. 1A, the classic worm transmission has two main components: the worm 100 and the gear 200. For clarity, the bearings and the casing that typically support these components are not represented in the schematic. The distance between their axes is specified by aD. In the non-backdrivable configuration (small worm pitch) the input is the rotational motion of the worm (α) and the output is the rotation of the worm gear (γ). When the worm turns, its spiral shaped teeth tangentially slide on the gear teeth, like a continuous wedge. The contact region shifts axially thus engaging the teeth of the gear and rotating it.
During motion there is a continuous sliding of surfaces on the worm and gear teeth. For this, the transmission requires sustained lubrication as well as the use of dissimilar (friction-paired) materials for gear and shaft that are paired for minimal friction and reduced wear.
FIG. 1B shows a normal view of the worm and gear teeth at the region of contact. The cross section of the worm tooth is trapezoidal while the gear is involute. Depending on the distance between the axes of the worm and gear (aD) there is either clearance or interference between the teeth. Ideally the distance aD would be set so that this gap was zero. In practice this perfect condition is impossible to achieve due to misalignment and manufacturing errors. Moreover, this is impossible to maintain due to the wear of the surfaces under sliding friction. Therefore, in case a clearance is present, the transmission presents backlash, the worm gear plays. If interference is present, the friction forces are highly increased and the transmission is either blocked or very inefficient creating premature wearing. Furthermore, it is common that the worm and the gear are not perfectly mounted on their shafts so that they are eccentric. This causes the gear to present variable clearance or interference at different angular positions, consequently the transmission does not exhibit uniform performance, is kinematically inconsistent, and imprecise.
Thus, although the classic worm transmission is simple, non-backdrivable, and may be miniaturized, its sliding friction causes reduced efficiency and kinematic inconsistency. In the case of the screw mechanism, a similar problem has been resolved by creating a ball-screw mechanism that uses spherical balls to replace the sliding friction by rolling friction, thus rendering a highly efficient non-backlash mechanism. In the present invention the ball rolling principle is implemented on a worm mechanism, hence creating the ball-worm transmission.